1. Field of the Disclosure
The present disclosure relates to methods of document creation. More specifically, the present disclosure is directed to a method and apparatus for indirectly measuring the flatness of a substrate media upon which an image is printed by an ink jet print system.
2. Brief Discussion of Related Art
In certain printers using ink jet technology, it is expected that inks, e.g., solid inks, UV inks, aqueous inks, and functional inks including those used in 3D printing application or printed electronics, among others, will be jetted directly onto substrate media, often a cut sheet. A critical parameter in this printing process is the size of the printhead-to-media gap. In certain current technology, the gap is set as small as 0.5 mm in order to minimize the pixel placement errors due to misdirected jets. For other printheads, for example those having relatively higher drop velocity, it is possible that the gap can be opened to between about 0.75-1.0 mm.
For accurate pixel placement and color registration, it is desired to keep the printhead-to-media gap within a +/−0.1 mm range about the nominal. To avoid printhead front face damage, under no circumstances is the media allowed to “close the gap”, i.e., to contact the printhead. Both vacuum and/or electrostatic escort belt, drum or plate technology are employed to hold cut sheets of substrate media sufficiently flat. However, these tight printhead-to-media tolerances pose a challenge for any cut sheet printer, since the cut sheet body is generally not perfectly flat.
One solution to the problem of upcurl is that a cut sheet printer may have a precurler subsystem which biases all sheets into a downcurl mode. However, sheets may not be held sufficiently flat in the printing zone, to the extent that a shutdown of the printer would be necessary to avoid the media contacting the printheads.
Therefore, the cut sheet media is subjected to a hold-down force, for example of electrostatic and/or vacuum pressure origin, as it is carried by the escort belt through a printing zone. Moreover, the images are formed in a dynamic process, whereby the substrate media is carried in a continuous and preferably high quality motion past the ink jet aperture array. Under the influence of the hold-down force, the substrate media is presumed to be held perfectly flat against the surface of the escort belt, drum or plate.
A known difficulty in the technology is that the presumption of media flatness past the ink jet aperture printing array may not hold. In particular, media supplied with a “pre-curl”, or curvature which biases the media to form an arc that would tend to lift the center of the substrate media off the surface of the escort belt, drum or plate, supported by its edges, i.e., a lead edge (LE) and a trail edge (TE), considered in the process direction. This bias is then overcome by the applied hold-down force. However, without the ability to measure the flatness of the media in the printing zone, the necessary amount of hold-down force is subject to some speculation. In order to overcome this, the hold-down force is intentionally over-applied, which is at least a source of inefficiency.